On Mon, Mar 14, 2022 at 06:38:31PM +0100, Claudio Fontana wrote:
On 3/14/22 6:17 PM, Daniel P. Berrangé wrote:
On Sat, Mar 12, 2022 at 05:30:01PM +0100, Claudio Fontana wrote:
the first user is the qemu driver,
virsh save/resume would slow to a crawl with a default pipe size (64k).
This improves the situation by 400%.
Going through io_helper still seems to incur in some penalty (~15%-ish) compared with direct qemu migration to a nc socket to a file.
Signed-off-by: Claudio Fontana <cfontana@suse.de> --- src/qemu/qemu_driver.c | 6 +++--- src/qemu/qemu_saveimage.c | 11 ++++++----- src/util/virfile.c | 12 ++++++++++++ src/util/virfile.h | 1 + 4 files changed, 22 insertions(+), 8 deletions(-)
Hello, I initially thought this to be a qemu performance issue, so you can find the discussion about this in qemu-devel:
"Re: bad virsh save /dev/null performance (600 MiB/s max)"
https://lists.gnu.org/archive/html/qemu-devel/2022-03/msg03142.html
RFC since need to validate idea, and it is only lightly tested:
save - about 400% benefit in throughput, getting around 20 Gbps to /dev/null, and around 13 Gbps to a ramdisk. By comparison, direct qemu migration to a nc socket is around 24Gbps.
restore - not tested, _should_ also benefit in the "bypass_cache" case coredump - not tested, _should_ also benefit like for save
Thanks for your comments and review,
Claudio
diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c index c1b3bd8536..be248c1e92 100644 --- a/src/qemu/qemu_driver.c +++ b/src/qemu/qemu_driver.c @@ -3044,7 +3044,7 @@ doCoreDump(virQEMUDriver *driver, virFileWrapperFd *wrapperFd = NULL; int directFlag = 0; bool needUnlink = false; - unsigned int flags = VIR_FILE_WRAPPER_NON_BLOCKING; + unsigned int wrapperFlags = VIR_FILE_WRAPPER_NON_BLOCKING | VIR_FILE_WRAPPER_BIG_PIPE; const char *memory_dump_format = NULL; g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(driver); g_autoptr(virCommand) compressor = NULL; @@ -3059,7 +3059,7 @@ doCoreDump(virQEMUDriver *driver,
/* Create an empty file with appropriate ownership. */ if (dump_flags & VIR_DUMP_BYPASS_CACHE) { - flags |= VIR_FILE_WRAPPER_BYPASS_CACHE; + wrapperFlags |= VIR_FILE_WRAPPER_BYPASS_CACHE; directFlag = virFileDirectFdFlag(); if (directFlag < 0) { virReportError(VIR_ERR_OPERATION_FAILED, "%s", @@ -3072,7 +3072,7 @@ doCoreDump(virQEMUDriver *driver, &needUnlink)) < 0) goto cleanup;
- if (!(wrapperFd = virFileWrapperFdNew(&fd, path, flags))) + if (!(wrapperFd = virFileWrapperFdNew(&fd, path, wrapperFlags))) goto cleanup;
if (dump_flags & VIR_DUMP_MEMORY_ONLY) { diff --git a/src/qemu/qemu_saveimage.c b/src/qemu/qemu_saveimage.c index c0139041eb..1b522a1542 100644 --- a/src/qemu/qemu_saveimage.c +++ b/src/qemu/qemu_saveimage.c @@ -267,7 +267,7 @@ qemuSaveImageCreate(virQEMUDriver *driver, int fd = -1; int directFlag = 0; virFileWrapperFd *wrapperFd = NULL; - unsigned int wrapperFlags = VIR_FILE_WRAPPER_NON_BLOCKING; + unsigned int wrapperFlags = VIR_FILE_WRAPPER_NON_BLOCKING | VIR_FILE_WRAPPER_BIG_PIPE;
/* Obtain the file handle. */ if ((flags & VIR_DOMAIN_SAVE_BYPASS_CACHE)) { @@ -463,10 +463,11 @@ qemuSaveImageOpen(virQEMUDriver *driver, if ((fd = qemuDomainOpenFile(cfg, NULL, path, oflags, NULL)) < 0) return -1;
- if (bypass_cache && - !(*wrapperFd = virFileWrapperFdNew(&fd, path, - VIR_FILE_WRAPPER_BYPASS_CACHE))) - return -1; + if (bypass_cache) { + unsigned int wrapperFlags = VIR_FILE_WRAPPER_BYPASS_CACHE | VIR_FILE_WRAPPER_BIG_PIPE; + if (!(*wrapperFd = virFileWrapperFdNew(&fd, path, wrapperFlags))) + return -1; + }
data = g_new0(virQEMUSaveData, 1);
diff --git a/src/util/virfile.c b/src/util/virfile.c index a04f888e06..fdacd17890 100644 --- a/src/util/virfile.c +++ b/src/util/virfile.c @@ -282,6 +282,18 @@ virFileWrapperFdNew(int *fd, const char *name, unsigned int flags)
ret->cmd = virCommandNewArgList(iohelper_path, name, NULL);
+ if (flags & VIR_FILE_WRAPPER_BIG_PIPE) { + /* + * virsh save/resume would slow to a crawl with a default pipe size (usually 64k). + * This improves the situation by 400%, although going through io_helper still incurs + * in a performance penalty compared with a direct qemu migration to a socket. + */ + int pipe_sz, rv = virFileReadValueInt(&pipe_sz, "/proc/sys/fs/pipe-max-size");
This is fine as an experiment but I don't think it is that safe to use in the real world. There could be a variety of reasons why an admin can enlarge this value, and we shouldn't assume the max size is sensible for libvirt/QEMU to use.
I very much suspect there are diminishing returns here in terms of buffer sizes.
64k is obvious too small, but 1 MB, may be sufficiently large that the bottleneck is then elsewhere in our code. IOW, If the pipe max size is 100 MB, we shouldn't blindly use it. Can you do a few tests with varying sizes to see where a sensible tradeoff falls ?
Hi Daniel,
this is a very good point. Actually I see very diminishing returns after the default pipe-max-size (1MB).
The idea was that beyond allowing larger size, the admin could have set a _smaller_ pipe-max-size, so we want to use that in that case, otherwise an attempt to use 1MB would result in EPERM, if the process does not have CAP_SYS_RESOURCE or CAP_SYS_ADMIN. I am not sure if used with Kubevirt, for example, CAP_SYS_RESOURCE or CAP_SYS_ADMIN would be available...?
So maybe one idea could be to use the minimum between /proc/sys/fs/pipe-max-size and for example 1MB, but will do more testing to see where the actual break point is.
That's reasonable. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|